Method to Evaluate the Appropriateness of Introducing a High Speed Data Transmission System Using Metallic Telecommunication Lines onto Railways

Transcription

1 PAPER Method to Evaluate the Appropriateness of Introducing a High Speed Data System Using Metallic Telecommunication Lines onto Railways Keiichi TAKEUCHI Assistant Senior Researcher, Kazuki NAKAMURA Senior Researcher, Kunihiro KAWASAKI Laboratory Head, Telecommunications and Networking Laboratory, Signalling and Transport Information Technology Division Daisuke YAMAGUCHI Yusuke KAWAMURA Researcher, Researcher, Telecommunications and Networking Laboratory, Signalling and Transport Information Technology Division High speed data transmission systems using metallic telecommunication s such as xdsl (Digital Subscriber Line) are being introduced into railway systems as a substitute to analog systems where adoption of optical carrier is not cost effective and to supplement optical carrier systems between a station with optical terminal equipment, and a station without optical terminal equipment. Research carried out so far has produced a method for assessing the suitability of coupling high speed data transmission systems for railways with normalstate telecommunication s. This study proposes an improved evaluation method, which considers the influence of the disturbances on the electrical characteristics of telecommunication s, and delay time and jitter when transmission systems are connected in tandem. Keywords: metallic telecommunication, high speed data transmission, xdsl, disturbance 1. Introduction Progress is being made in introducing high-speed data transmission xdsl technology as a substitute for analog carrier communication, or to complement optical carrier communication. Introduction of high-speed data transmission systems requires taking into consideration the influence of crosstalk between the high-speed data transmission system and the existing transmission systems bundled into the same cable, and the influence of noise in metallic telecommunication s along railways [1, 2]. The proposed evaluation method which would currently be applied to s planned to be introduced into a railway environment determines whether the received S/N signal in the planned fulfills transmission quality requirements [3, 4]. However, this method is based on the premise that the highspeed data transmission system will be added to telecommunication s in a normal state. Moreover, high-speed data transmission systems are often connected in tandem with the telecom s, whereby a disturbance in any section may impact the whole transmission system. netheless, measuring this impact in real terms to ascertain the level of influence before introducing the high-speed data transmission system is too problematic, hence the need for a prediction method to estimate the scale of the impact. Consequently, a method to assess suitability of introducing high-speed data transmission system using xdsl was developed and is proposed for predicting S/N when a disturbance appears on the, producing estimations of the transmission delay time where there are tandem connections. This report describes the out of the proposed evaluation method, and introduction tool to assist in evaluation based on the proposed method. 2. Influence of disturbances on electrical characteristics 2.1 Line disturbance indices Indicators pointing to a main disturbance in metallic telecommunication s include loose connections, disconnections, inter alia. Line disturbances are recognized when any one of various electrical characteristic indices of the metallic railway telecommunication cables falls below the standard value. These indices are shown in Table 1. The existing method includes indices corresponding to disturbance phenomena leading to an increase in conductor resistance, change in characteristic impedance, increase in loss, and increase in noise, as shown in Table 1. Index Insulation resistance Table 1 Index of disturbance Standard value Over 5M Ω km (When cable length is under 5km, over 1M Ω ) Conductor resistance.9mm wire, under 29. Ω /km Unbalanced conductor resistance Characteristic impedance Under 2% N/A.9mm wire 42 Ω (loaded circuit 138 Ω ) Evaluation by the existing method N/A OK OK Line loss.9mm wire,.8db/km OK Near-end crosstalk loss Over 65dB OK Line noise Under 1.mV OK QR of RTRI, Vol. 54,. 2, May

2 Indices corresponding to faulty insulation resistance and unbalanced conductor resistance however, are not included. Influence from these two factors therefore form the object of the present research. 2.2 Influence of disturbances Under normal conditions R 1 Telecommunication R 2 This section examines and describes influences of faulty insulation resistance and conductor resistance imbalances in the telecom on the electrical characteristics of the high-speed data transmission system to be introduced. Faulty insulation resistance occurs when there is a fall in insulation resistance R L1-L2 between wire and wire of the metallic telecommunication, as shown in Fig. 1. It is assumed that the fall of this insulation resistance appears as an increase of loss. A circuit model was built to calculate the rise in loss when insulation resistance falls. Following on, the rise in loss for every insulation resistance value was calculated using the electric circuit simulator (EMTP) based on the model. Figure 2 shows the results of these calculations. Figure 3 on the other hand illustrates the unbalanced conductor resistance which occurs when there is a difference between the conductor resistance R 1 of wire and the conductor resistance R 2 of wire constituting the metallic telecommunication. The extent of the difference, i.e. the unbalance factor of conductor resistance is calculated by (1). Unbalance factor (%) = R1 R2 2 1 (1) R + R 1 2 It is assumed that an increase in conductor resistance of only one wire in a metallic telecommunication ac- Under normal conditions With presence of a disturbance Increased amount of loss (db) Fig Telecommunication Telecommunication R L1 L2 R L1 L2 Fall ascribed to presence of a disturbance (instead of falling according to disturbance) Fig. 1 Faulty insulation resistance Insulation resistance(ω) Rise in loss in case of faulty insulation resistance (Example of a calculation result using EMTP) R 1 =R +R X L R R X 1 With presence Telecommunication of a disturbance R 2 Fig. 3 Unbalanced conductor resistance Increased amount of loss(db) Unbalance factor of conductor resistance(%) Fig. 4 Rise in loss in case of unbalanced conductor resistance (Example of survey result) cording to unbalanced conductor resistance appears as an increase in loss. Insight into the relationship between the increment of conductor resistance in only one wire and the increase in loss was obtained through a survey. Figure 4 gives an example of the survey results. 2.3 Influence of disturbance on adjoining Disturbances on a telecommunication will influence the electrical characteristics of the adjoining in the same cable. The present section describes this impact. Figure 5 illustrates the cross-talk produced on an adjoining due to a fall in insulation resistance between it and the telecommunication. A circuit model was built to calculate the increase in near-end-crosstalk noise. The rise in near-end-crosstalk noise intensity in the adjoining due to change in insulation resistance, was calculated using the electric circuit simulator. Figure 6 shows the result of this calculation. Figure 7 shows that the power lost in the with the disturbance from the unbalanced conductor resistance appears as crosstalk noise in the adjoining. Following on, a model to calculate the increase in near-end-crosstalk noise was built. Increases in near-end-crosstalk noise intensity from increases in unbalance factor of conductor resistance were calculated using the electric circuit simulator. An example of the calculation result is shown in Fig QR of RTRI, Vol. 54,. 2, May 213

3 Adjoining Fig. 5 Influence of faulty insulation resistance on adjoining Increased amount of the near-end-crosstalk noise intensity(db) Fig. 6 Rise in near-end-crosstalk due to faulty insulation resistance R X Obstacle Telecommunication Adjoining Fig. 7 L 3 L Telecommunication Fall of insulation resistance Telecommunication Appearing as crosstalk noise due to the fall of insulation resistance Insulation resistance(ω) Appearing as crosstalk noise due to the loss of power L 3 Telecommunication L 4 Influence of unbalanced conductor resistance on an adjoining 3. Impact of disturbances on delay time and jitter 3.1 Delay time and jitter in case of tandem connections Long distance transmission systems constructed using xdsl comprise several sections connected in series, as shown in Fig. 9. Conventional methods for to evaluate Increased amount of near-end-crosstalk noise intensity(db) Unbalance factor of conductor resistance(%) Fig. 8 Rise in near-end-crosstalk due to conductor resistance imbalance (Example of calculation for a length of 5 m) the suitability of introducing high-speed data transmission systems, are based on calculating and comparing the S/N for each section. If a terminal has permissible delay time T or jitter J as shown in Fig.9, however, the aforementioned method is insufficient for this type of evaluation without the additional estimation of the total time and jitter end to the end. Consequently it is necessary to clarify the relationship between the delay time of each section T ab, T bc, T cd and the total delay time from station A to station D, and the relationship between the jitter of each section J ab, J bc, J cd and the total jitter from station A to station D. Where the S/N satisfies the required set transmission speed, it is assumed that transmission equipment is operating according to specification; further values are then specified for delay time and jitter to be applied to the transmission equipment. Experiments results demonstrated that total delay time could be calculated by summing up the delay time of each section when SHDSL transmission equipment was connected in tandem. The results of this experiment are shown in Fig. 1. When different sets of transmission equipment possessing the same specifications are connected in tandem, total delay time under normal conditions can be calculated by (specification value) x (number of section), as shown in Fig. 1. Next, jitter was examined. Figure 11 shows jitter between the sets of transmission equipment. When the time difference between the point at which a certain packet is transmitted from the transmission equipment A and the moment it reaches the transmission equipment B is considered as delay time, when delay time of the packet reaching the transmission equipment B is a = b = c, jitter will not oc- Delay time T ab Delay time T bc Delay time T cd Jitter J ab Jitter J bc Jitter J cd Station A Station B Station C Station D Terminal Terminal Required total delay time T Required total jitter J T Tab Tbc T and J J cd ab Jbc J cd are fulfilled? Fig. 9 Configuration of a long distance transmission system example connected in tandem QR of RTRI, Vol. 54,. 2, May

4 Round trip delay time(ms) Fig kbps 4Wire 1536kbps 2Wire 768kbps 4Wire 768kbps 2Wire Number of section The experimental results for delay time when transmission equipment possessing the same specifications are connected in tandem equipment A Packet Packet transmission a b c t 1 t 2 equipment B ed by disturbances. For this reason, when a disturbance occurs in a certain section, S/N falls, preventing S/N from satisfying the value required for the specified access speed, and transmission speed may then fall in the section. It is possible that the fall of the transmission speed in some sections affects the total delay time and jitter in the whole transmission system. Consequently, investigations were carried out in order to find a means to calculate the delay time and the jitter when a disturbance occurs. Figure 12 shows the calculation model for delay time and jitter for when a disturbance occurs. In sections where the S/N no longer meets the set speed for the whole transmission system, the transmission speed falls or the is disconnected. When the is disconnected, delay time becomes infinite, and it is judged that introduction of a high-speed data transmission system is impossible. When transmission speed falls, the number of outbound packets decreases to below the number of incoming packets in the emission transmission equipment buffer, and packets accumulate in the buffer. The interval from the time when the last packet (the outbound packet Z in Fig. 12) in a buffer is transmitted from the emitter side transmission equipment up to the moment it reaches the receiver transmission equipment indicates the maximum delay time in the section where a disturbance has appeared. Moreover, maximum jitter can be deduced from the difference between the maximum and minimum delay times. Fig. 11 Out of jitter between sets of transmission equipment cur because t 1 = t 2. In reality, however, delay times are different and this translates into jitter. When the delay time a turns out to be the minimum delay time between transmission equipment A and B and the delay time b turns out to be the maximum delay time between transmission equipment A and B, t 1 becomes the maximum jitter between transmission equipment A and B. 4. Method to evaluate the appropriateness of introducing a high speed data transmission system Based on the method proposed above, which estimates total delay times and jitter when a disturbance occurs on the, another method was examined with a view to evaluating the suitability of introducing high-speed data transmission systems. 4.1 Basic concepts underpinning the evaluation method 3.2 Calculation model of Delay time and jitter in case of disturbance As explained in section two, the electrical characteristics of the signal level (S) and noise level (N) are affect- tation A From of normal transmission speed equipment of sending side Past methods assume that the S/N which satisfies the transmission quality (frame wastage rate: FLR) required for a certain transmission speed is known. This is done by comparing and checking that the receiving S/N from the data transmission to which the high-speed data trans- speed falls due to the fall of S/N equipment of receiving side Modulating Demodulation Buffer equipment equipment Buffer Z Y X 2 1 Cancel Packet Packet Buffer of transmission equipment of sending side Number of inflow packet > Number of sending-out packet Fig. 12 Calculation model of delay time and jitter in case of disturbance Station B To of normal transmission speed 1 QR of RTRI, Vol. 54,. 2, May 213

5 Input item Cable characteristic loss crosstalk noise etc equipment characteristic required S/N Power spectrum Delay time and jitter specification value etc distance Examining the possibility of transmission on a to which introduction of high-speed data transmission system is planned Characteristic of loss Calculation of the delay time at the normal time Power spectrum transmission equipment Calculation of S/N Calculation of S/N at the time of obstacle Larger than required S/N? Delay time and jitter smaller than allowable value? Line noise Calculation of the delay time at the time of a obstacle Line disruption input item Case of bad insulation Selection of a bad insulation part and a insulation resistance value (a) E(or E) (b) (c) L adjoining Case of unbalance R:roop resistance R 1: resistance R 2: resistance Unbalance rate=(r 1+R 2)*2*1/R Calculation of the increase in the amount of loss is possible. transmission is impossible Examining the possibility of transmission on a adjoining the planned for introduction Characteristic of loss Power spectrum transmission equipment Line noise Crosstalk noise Calculation of S/N Calculation of S/N at the time of obstacle Calculation of the increase in the amount of crosstalk noise Larger than required S/N? Output item Planned for introduction Adjoining S/N speed Maximum delay time Maximum jitter Evaluation of transmission properties Calculation of the delay time at the normal time influence on an adjoining Delay time and jitter smaller than allowable value? All s can be transmitted Calculation of the delay time at the time of a obstacle Influence on an adjoining Existing method Parts of new proposed method = boxes outd in bold Fig. 13 Flow chart illustrating method used to evaluate appropriateness of introducing a high-speed data transmission system mission system is to be added, is larger than the required S/N. The suitability of introducing the high speed transmission is then judged on this basis. The proposed evaluation method in this report uses the same approach to reflect the influence of disturbances on the receiving S/N. However, once introduction of the high-speed data transmission system is deemed feasible on the basis of S/N evaluation, the new method goes on to calculate the delay time and the jitter for the whole long-distance data transmission system which are then compared with permissible values to assess the final suitability of its introduction. A flow chart illustrating the proposed method is shown in Fig S/N evaluation method for when a disturbance occurs S/N is calculated from receiving signal strength (S) and noise field intensity (N). The increase in the amount of loss degrades S, and the increase in near-end-crosstalk noise increases N. As explained in section two poor insulation resistance and conductor resistance imbalances, cause an increase in both loss in the section where the disturbance has occurred, and in near-end-crosstalk noise in the adjoining circuit. It is therefore possible to predict the receiving S/N when a disturbance appears. by comparing the increases in loss, and in near-end-crosstalk noises with the receiving S/N under normal circumstances. 4.3 Technique for evaluating delay time and jitter When the receiving S/N satisfies the required S/N value, the delay time and jitter of the whole transmission system are calculated from the sum of the specified values for each section. Suitability of introducing the high-speed data transmission system is established according to whether this result is smaller than the allowable delay time and jitter in the transmission system. When the receiving S/ N does not satisfy the required S/N value for transmission speed in a part the section because of a disturbance, this means the desired transmission quality cannot be met. If planned transmission still goes ahead, it is regarded that the speed falls to a level which still fills the required S/N value which was obtained beforehand, based on the receiving S/N. Even though transmission speed falls in one section alone, the delay time and jitter of the whole transmission system increase. The maximum delay time and jitter in QR of RTRI, Vol. 54,. 2, May

6 the section which does not meet the required S/N value are calculated using the model explained in Section 3.2 and are used to replace the section s specified delay time and jitter values; the total delay time and jitter for the whole transmission system is then obtained by summing up the values in each section and is compared with the overall required delay time and jitter. 5. Tool to support the evaluation for determining the appropriateness of introducing a high speed data transmission system The proposed evaluation requires many input parameters, and is very time consuming and complicated if calculations are done by hand. A support tool using a Windows application was therefore developed to reduce this workload. The developed application has a user interface for inputting the parameters required for evaluation. The input screens are explained below. 5.1 Main evaluation screen The main evaluation screen, is used to input the section under evaluation, the transmission system to be introduced into each metallic telecommunication cable, etc. In order to evaluate the impact of disturbances, the screen to input the details of the disturbance is reached by checking the relevant box. Permissible delay times and jitter for the whole transmission system, margins, etc. are input via this main screen. Calculation results for S/N and evaluation results are also displayed on this screen. This main screen is shown in Fig. 14. Introduction propriety evaluation main screen The detailed input of a obstacle is attained by checking a predetermined check box. Table 2 Main input items for transmission equipment characteristics Input item Contents of input Unit:ms Delay time Delay time at the set-up transmission speed (Specification value) Unit:ms Jitter Jitter at the set-up transmission speed (Specification value) Number of packets which can be accumulated Buffer of equipment in a transmission equipment buffer (Specification value) Unit:dB S/N - transmission speed Required S/N to transmission speed erage value based on past measurement data is set as the default value, other data from surveys may be entered for evaluation under this section Line disturbance input screen This section is for entering items related to poor insulation resistance and conductor resistance imbalances. The relevant items are listed in Tables 3 and 4. Table 3 Line disturbance input items (Poor insulation resistance) Input item Contents of input Disturbance number Insulation resistance Line loss increment Crosstalk increment Line number where disturbance occurrence is supposed Unit:kW Insulation resistance value of an disturbance part In case of inputting loss increment In case of inputting crosstalk increment Fig. 14 Main screen for suitability evaluation 5.2 Input parameters equipment characteristics input screen The characteristics of the transmission system for introduction are input via the relevant input screen. The main items to be entered under this section are shown in Table Cable characteristics input screen This screen is for entering loss and crosstalk noise characteristics of the to be evaluated. Although the av- Table 4 Line disturbance input items (Conductor resistance imbalance) Input item Disturbance number Circuit resistance 1 and 2 Conductor resistance rate of imbalance Line loss increment Crosstalk increment Contents of input Line number where disturbance occurrence is supposed Unit:W Conductor resistance value of L1 and L2 Refer to (1) in Section 2.1 for the formula In case of inputting loss increment In case of inputting crosstalk increment 6. Verification of the proposed method In order to verify the proposed evaluation method for evaluating properties of the high-speed data transmission system, laboratory and field tests were carried out on a section where SHDSLs were connected in tandem, as shown in Fig. 15. In both tests a pure resistance was employed to imitate a disturbance in the circuit, and transmission 12 QR of RTRI, Vol. 54,. 2, May 213

7 The resistance which imitates the disruption is inserted. One-step temporary construction 6km 9km 9.1km 9.3km 2.6km SHDSL SHDSL SHDSL SHDSL SHDSL SHDSL SHDSL SHDSL SHDSL SHDSL SHDSL SHDSL A station B station C station D station E station F station compartmen Fig. 15 Composition of a transmission test Increased amount of the near-end-crosstalk noise intensity(db) is possible is impossible Laboratory experiment Field test (Predicted from a conductor resistance value) The prediction result of a technique Unbalance factor of conductor resistance(%) a result, a method was proposed to evaluate the suitability of introducing a high-speed data transmission system. The method was based on predicting S/N for when a disturbance occurs and also allowed calculation of the subsequent delay time and jitter caused by the fall in S/N quality. A support tool for this method was also developed on the basis of this method. The tool can be used not only for assessing the impact of disturbances for planned introduction of high-speed data transmission systems but can also applied for evaluations under normal circumstances, can reduce work involved in performing such assessments and be used on existing s. Model for calculating unbalance factor for conductor resistance Fig. 16 Example of verification of the evaluation method quality was measured. An example of the results obtained from these verifications in the case of assuming conductor resistance imbalance is shown in Fig. 16. The figure shows the where transmission is possible or and where it is not according to the test and prediction based on the evaluation method with O and X respectively. Figure 16 also shows that observed results and predictions generally agreed confirming the validity of the proposed method. 7. Conclusions This paper considered the influence produced by disturbances on a telecommunications to which a highspeed data transmission system was due to be introduced, and the impact of such disturbances on adjoining s. As References [1] Shindo, M., Nakamura, K., Seki, K., An Examination of High Bit Data by Metallic Cables for Railways, Quarterly Report of RTRI, Vol. 43,.4, pp , 22. [2] Shindo, M., Nakamura, K., Seki, K., An Estimation Method for Introducing xdsl Technology into Railways, RTRI Report, Vol. 17,.6, pp , 23 (in Japanese). [3] Takeuchi, K., Seki, K., An Estimation Method of the Characteristic of High-speed Data Lines Using Metallic Cables along Railway Lines, RTRI Report, Vol. 2,.1, pp , 23 (in Japanese). [4] Takeuchi, K., An Estimation Method of the Possibility of Constructing High-speed Data Lines Using Metallic Cables along Railway Lines, 8 th World Congress on Railway Research (WCRR28), Seoul, Korea, May 18-22, 28. QR of RTRI, Vol. 54,. 2, May